1. Field of the Invention
The present invention relates to a liquid crystal display device using an oxide semiconductor and a method for manufacturing the same.
2. Description of the Related Art
As typically seen in a liquid crystal display device, a thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon. A thin film transistor manufactured using amorphous silicon has low field effect mobility, but can be formed over a larger glass substrate. In contrast, a thin film transistor manufactured using polycrystalline silicon has high field effect mobility, but needs a crystallization step such as laser annealing and is not always suitable for a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique for manufacturing a thin film transistor using an oxide semiconductor and applying the thin film transistor to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor for an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.
The thin film transistor in which a channel formation region is formed using an oxide semiconductor has higher field effect mobility than a thin film transistor using amorphous silicon. The oxide semiconductor film can be formed by a sputtering method or the like at a temperature 300° C. or lower. Its manufacturing process is easier than that of a thin film transistor using polycrystalline silicon.
The oxide semiconductor is a transparent semiconductor which transmits light in a visible wavelength range; thus, it is said that use of the oxide semiconductor for a pixel of a display device makes it possible to provide a high aperture ratio.
Such an oxide semiconductor is expected to be used for forming a thin film transistor on a glass substrate, a plastic substrate, or the like, and to be applied to a display device.